There are a group of breast tumors that have a significantly worse prognosis than the others, the so-called triple negative tumors (ER-ve, PR-ve, HER2-). One reason for their poor prognosis is a lack of targeted therapies (they are non-responsive to tamoxifen or herceptin). In this application, it is postulated that carcinogen-induced breast tumors in mice are a good model of triple negative breast cancer. By transcriptional profiling, they cluster in a mouse subgroup (including tumors from BRCA1, p53 and Wnt transgenic mice) that resembles the profile of basaloid, triple negative group from women. They have markers of bilineal differentiation, which implies the presence of a cancer stem cell. In the literature, there is some data to suggest that other types of cancer stem cells are relatively resistant to the standard chemo- and radiotherapeutic strategies, and that this could be the reason for tumor recurrence or non- responsiveness. This application seeks to resolve a classic dilemma in this field, which is whether the tumor precursor cells are drawn from normal stem cells in the breast. If the process of recruitment is better understood, it may be possible to understand not only how to prevent precursor transformation, but also whether the cancer stem cells share more properties with normal stem cells (or have acquired just one specific function that confers immortal growth potential). A specific mouse strain with a mutant allele of a heparan sulfate proteoglycan, syndecan1 (Sdc1), is dramatically resistant to carcinogen- and oncogene- induced tumors and is suspected to have a key deficiency in the pathogenic activation of stem/progenitor cells. This mouse strain therefore represents a tool that can help to pinpoint a highly susceptible stage of cancer initiation. In this application, the Aims include the elucidation of the origin of carcinogen-induced tumors (using a newly developed mouse strain based on the discovery of LRP5 as a mammary stem cell biomarker), the discovery of the cellular mechanism of tumor protection in Sdc1-/- mice, a description of the specific response of stem, progenitor and differentiated cell fractions to genotoxic carcinogen administration, and the elucidation of the molecular pathways that underlie their resistance to tumor development. The initiating events that start this tumor process are currently not at all understood, and the analysis of this mouse strain represents a unique opportunity for discovery. PUBLIC HEALTH RELEVANCE: A specific strain of mouse (carrying a mutation in the syndecan-1 gene, a cell surface heparan sulfate proteoglycan) is dramatically resistant to tumors that develop after treatment with carcinogens. There is evidence to suggest that the precursor cells for these tumors originate in the normal stem cell compartment, and that they are mutated to become "cancer stem cells". Analysis of the response of breast tissue from these mice during carcinogen administration can pinpoint the factors that regulate the recruitment of mammary stem cells to tumors, and advise on how to reduce tumor load after carcinogen exposure.